1. Field of the Invention
The present invention relates to a method for allocating a call received from such a network as an integrated services digital network (ISDN) to a plurality of extension telephone sets provided in private branch exchange connected to the ISDN as well as a private branch exchange having a function of automatically allocating the call to desired one of the extension telephone sets. More particularly, the present invention relates to an extension telephone allocating method as well as an arrangement of a private branch exchange, which allows direct sending of a call received from the network to a desired one of the extension telephone sets.
2. Description of the Related Art
Generally speaking, an electronic private branch exchange connected to the ISDN or the like is arranged so that, when the exchange receives an incoming call from a central office line (public line), users can arbitrarily set a relay board or an extension number (indicative the telephone number of an extension telephone set, which is also referred to as a directory number (DN) as its target call receiver with respect to each of port numbers of the central office line. The call allocating system to the extension telephone sets is also provided with various service functions including an automatic call distribution (ACD) and dial-in.
In the ACD system, for example, at the time of setting up the system (at the time of setting initial conditions), extension numbers indicative of the telephone numbers of a plurality of extension telephone sets are previously set in an ACD group as associated with the extension telephone sets and the ACD group is set as a call receiver for a specific central-office line port number. With such an arrangement, when an incoming call appears at the specific central-office line port number, the incoming call can be sent to one of the extension telephone sets belonging to the ACD group being assigned as the call receiver. In this case, the private branch exchange selectively controls the call receiver in such a manner that the received call can be uniformly distributed and sent to the respective extension telephone sets belonging to the ACD group.
In the ACD system, however, though it is possible to assign a call receiver in a group unit as mentioned above, it is not always possible that a received call is sent to a desired call receiver. For this reason, when the call is sent to one of the extension telephone sets other than the desired one, this requires a wrong call receiver to conduct the following call informing work. That is, the wrong call receiver checks the extension number of the extension telephone set which the caller assigns and transfers the call to the right extension telephone corresponding to the right extension number. Only after the troublesome call informing work, the caller can talk with the desired party.
In the dial-in system, on the other hand, the users previously set specific extension numbers for respective central-office line port numbers. And when the system receives an incoming call from the central office line, the incoming call is directly sent to an extension telephone set having a specific extension number.
In this way, in the dial-in system, it is possible to specify the call receivers to some extent. However, because the number of extension telephone sets is usually larger than the number of central office lines in a private branch exchange, it is impossible for the central office lines to be allocated to all the extension telephone sets in a 1:1 relation therebetween. Thus, even in the dial-in system, when the system receives an incoming call, it is not always possible that the received call is sent to a desired call receiver. For this reason, when the call is sent to one of the extension telephone sets other than the desired one, this requires the troublesome call informing work of the wrong call receiver as mentioned above.
As explained above, in the electronic private branch exchange, the call received from the central office line can be sent to any of the extension telephone sets belonging to the specific ACD group or to the preset extension telephone sets for each central-office line port number, but the call cannot be always sent to the desired extension telephone set. In many cases, this involves such troublesome call informing work as mentioned above. In addition, the caller must wait during the call informing work, while the wrong call receiver must troublesomely check the extension number of the right extension telephone set specified by the caller and transfer the call to the right extension telephone set. For this reason, improvements in the extension telephone allocating method for the call from the central office line have been in strong demand.